This invention relates generally to the art of firing pulverized solvent refined coal in a burner for a boiler or the like.
Solvent refined coal, also known as SRC-I, is a low sulfur, low ash, solid fuel produced from coal and having a composition such that it could be used as a utility boiler fuel which can be burned under environmentally acceptable conditions. Solvent refined coal is produced by the dissolution and hydrogenation of pulverized coal in a process-derived solvent. The resulting process stream is flashed to remove hydrogen and like gases, processed via critical solvent, by filtration or by other methods to remove undissolved coal and ash, and then fractionated to separate byproduct gases and distillable liquids, recycle process solvent, and the solvent refined coal product. The solvent refined coal yield is the hydrocarbon fraction having a boiling point substantially greater than 850.degree. F. and generally represents 40 to 70 percent of the moisture ash free feed coal.
The solvent refined coal can be further hydroprocessed to produce more distillable liquids and another hydrocarbon fraction having a boiling point substantially greater than 850.degree. F. which is a product solvent refined coal.
As to its physical characteristics, solvent refined coal typically has a sintering temperature of 170.degree. F., a melting point of 284.degree. F., a specific gravity of 1.24 grams per cubic centimeter and a bulk density of 50 lbs per cubic foot as received. Also, solvent refined coal has a heating value which is considerably higher than that of coal, namely, about 15,600 Btu/lb under dry conditions. Also, solvent refined coal has a low melting point, in the range of 280.degree.-300.degree. F., and can become tacky at temperatures as low as 170.degree. F. The solvent refined coal after hydroprocessing normally has a lower melting point. Thus, for successful pulverization, the internal mill temperature should not exceed approximately 150.degree. F. In addition, to avoid fouling and coking of solvent refined coal on burner surfaces, cooling of the burner has heretofore been required.
In addition to its potential to be a very clean burning (low pollution) fuel, solvent refined coal (hereinafter SRC) could be fired in utility boilers originally designed to fire oil. Testing is under way by the U.S. Department of Energy to determine the suitability of solvent refined coal as a utility boiler fuel. Pursuant to this testing and in the contemplated use of SRC as a boiler fuel, the SRC is finely pulverized and then injected pneumatically into the furnace in a manner similar to that used for pulverized coal firing.
The firing of SRC involves several problems. Firstly, because SRC begins to soften at about 170.degree. F., there is a tendency for SRC to plug the tubes through which the SRC fuel is injected into the furnace. Another problem is that the solvent refining process removes only minor amounts of the nitrogen in the fuel wherefore the SRC must be fired in a manner such that excessive oxides of nitrogen (NO.sub.x), which is an air pollutant, are not produced.
The conventional approach to solving the first-mentioned problem is to provide for the water-cooling of the fuel injection tube so that the SRC fuel will not soften and adhere to it. The water-cooled fuel tube burners are sensitive to firing conditions such as fuel tube position and air swirl, and have a tendency to accummulate fuel deposits. Also, water-cooled burners are complicated to design, fabricate and maintain because of the numerous small water passages that are required. Further, to date, water-cooled burners have only been tested in wall-fired boilers (i.e., boilers in which the burners are positioned in rows on one or two furnace walls and fire in a direction perpendicular to the walls. At present, no water-cooled burners have been developed and proven for tangentially fired boilers in which the burners are positioned in the four corners of the furnace and fire in a direction just off the diagonals of the furnace to produce a fireball in the center of the furnace.